Blood flows into the brain primarily via the carotid arteries and the vertebral arteries. The Circle of Willis is a circular arterial structure in the brain that connects blood flowing in from the carotid arteries with blood flowing in from the basilar artery (which is fed by the vertebral arteries). Blood flows from the Circle of Willis into brain tissue via the anterior, middle, and posterior cerebral arteries. Many studies have shown that the Circle of Willis is incomplete in most people. A 1998 study of 150 healthy adult volunteers showed a complete Circle of Willis in only 42% of cases — more often complete in younger persons and females [RADIOLOGY; Krabbe-Hartkamp, MJ; 207:103–111 (1998)]. A slightly more encouraging 2002 study of 118 healthy volunteers in the 65–68 age group, showed 47% had a complete Circle of Willis [THE JOURNAL OF CARDIOVASCULAR SURGERY; Macchi, C; 43:887–890 (2002)]

For cryonics purposes, it has been believed that perfusion into the carotid arteries, but not into the vertebral arteries will result in incomplete perfusion of the brain if the Circle of Willis is not complete. In particular, if both posterior communicating arteries are missing, then perfusing only through the carotid arteries will result in no blood getting to parts of the brain supplied by the posterior cerebral arteries. Both posterior communicating arteries were missing in 11% of those in the 1998 study and in 14% of those in the 2002 study cited above.

Nonetheless, a 2008 study showing Circle of Willis complete in only 40% of 99 patients found no case of insufficient perfusion in functional tests of patients given unilateral cerebral perfusion. The authors concluded that “extracranial collateral circulation” provides an alternative pathway to the Circle of Willis for cerebral crossperfusion [EUROPEAN JOURNAL OF CARDIOTHORACIC SURGERY; Urbanski, PP; 33:402–408 (2008)]. Although persons with missing posterior communicating arteries could easily have pathways to opposite sides of the brain, other variants of Circle of Willis incompleteness would be expected to prevent perfusion across hemispheres.

21st Century Medicine (21CM) is a cryobiology research company whose core mission is to develop a cryopreservation protocol sufficiently benign that whole, donated human organs could be vitrified (stored below −130 degrees Celsius without ice formation) and rewarmed when needed for transplantation –an incredibly ambitious goal that has so far eluded medical researchers. 21CM’s scientists are the original pioneers of whole organ vitrification and have been diligently working on the technique for decades. A significant milestone of their progress toward that goal is their demonstration work on rabbit kidneys. Two of their most relevant publications are “Cryopreservation of organs by vitrification: perspectives and recent advances” (Fahy, Wowk, Wu, Phan, Rasch, Chang & Zendejas 2004) and “Physical and biological aspects of renal vitrification” (Fahy, Wowk, Pagotan, Chang, Phan, Thomson & Phan 2009). These papers are a fantastic resource for anyone interested in just how difficult it is to cryopreserve large organs (and by extension whole animals) for long-term storage with the intent on later recovery of biological function. In short, it is incredibly difficult but progress is slowly being made.

The state-of-the-art whole organ vitrification techniques developed by 21CM are the basis of the human cryopreservation protocols used by some cryonics companies (e.g. Alcor). This is made clear in Alcor’s 2004 publication in the New York Academy of Sciences “The Arrest of Biological Time as a Bridge to Engineered Negligible Senescence” (Lemler, Harris, Platt & Huffman 2004).

21CM has been an official competitor in our Brain Preservation Prize competition since 2012. They actually have two separate preservation techniques competing for our prize: the “straight” cryopreservation\cryonics technique described on this page, and a radically new technique called “Aldehyde Stabilized Cryopreservation” (ASC), described on a separate page,which they developed to overcome the “tissue shrinkage” issues described on this page and to optimize ultrastructure preservation at the expense of viability.